Cost: Estimates claim that 1/5 of the world’s food production is destroyed or contaminated by rats every year.

Major Concerns: Spreading diseases like the bubonic plague (via the rat flea), typhus, toxoplasmosis and trichinosis.

Although much of their threat has been remedied, the mere mention of rats conjures up horrific images for us. Their most gruesome hour came in the 14th Century when they carried the Black Plague across the globe. Beginning in Asia, the Black Death spread across Europe at a ferocious pace. Some estimates claim it wiped out up to two thirds of the European population and 75 million worldwide. Several theories abound about its origins but most claim it began when fleas contracted the disease from infected rats and, in turn, passed it onto humans.